Riser tensioning arrangement

ABSTRACT

An arrangement for transferring loads by means of one or more hydraulic cylinder-like actuators ( 19, 26, 40 ) in order to establish and maintain tension in a riser ( 1 ) of the type typically used in offshore petroleum production, where the riser ( 1 ) constitutes a connection between the sea bed ( 2 ) and an installation ( 4 ) on or near the surface of the sea ( 6 ), and where the required pressure difference between the pressure side(s) ( 18, 42 ) and the depressurised side(s) ( 17, 46 ) of the actuator(s) ( 19, 29, 40 ) is constituted by a hydrostatic pressure difference.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/NO02/00153, filed Apr. 23, 2002, whichinternational application was published on Nov. 7, 2002 as InternationalPublication WO 02/088512. The International Application claims priorityof Norwegian Patent Application 20012078, filed Apr. 27, 2001.

FIELD OF THE INVENTION

This invention regards an arrangement for establishing and maintainingtension in a riser of the type typically used for offshore petroleumproduction, where the riser constitutes a connection between the seabedand an installation on the surface of the sea.

BACKGROUND OF THE INVENTION

When establishing a petroleum well offshore by means of a drillingvessel, it is normal to set in place a riser extending between theseabed and the drilling vessel, relatively early in the drilling phase.Besides being used for controlling drill tools, the riser is also usedfor carrying drill fluid between the well and the drilling vessel.

A riser is ordinarily formed as a length of tubing telescopicallyconnected to the drilling vessel, where the lower portion of the riseris connected to seabed equipment. The dimensions and weight of the riserare considerable, and it is crucial to the function of the riser thattension be established and maintained in the riser, to ensure that theriser is subjected to insignificant compressive stresses only. If theriser is subjected to the load of its own mass, which may typicallyamount to several hundred tons, it will automatically break.

In order to establish and maintain sufficient tension in the riser,compensating means are normally connected between the drilling vesseland the upper portion of the riser. The compensating means typicallycomprise one or more hydraulic cylinders, where the pressure side of thecylinders are in communication with a hydraulic pump and hydraulicaccumulators through associated pipes and a system of valves.

The heave motion of the drilling vessel and changes in the sea level aretaken up by the telescope connection between the drilling vessel and theriser. The compensating means must be constructed so as maintain more orless constant tension in the riser, also during said motion, and theaccumulators of the compensating means are therefore designed to receiveand deliver the largest share of the pressure fluid that must be drainedand supplied to the compensating cylinders during the motion of thedrilling vessel.

It is obvious that hydraulic pumping and accumulator systems of the typereferred to here are both very extensive and complicated, while alsobeing costly both to procure, install and operate. Moreover, there is adanger that any interruption of power to the pumping system may after awhile entail a risk of insufficient supply of pressure fluid to thehydraulic compensating cylinders, whereby the riser may be damaged.

The object of the invention is to remedy the disadvantages of prior art.

SUMMARY OF THE INVENTION

The object is achieved in accordance with the invention by thecharacteristics stated in the description below and in the appendedclaims.

In a heave compensating means according to the invention, the requiredpressure difference between the two fluids acting on the two pistonsides of the compensating cylinders is effected by means of thehydrostatic pressure that exists under the surface of the sea.

In one embodiment in which the compensating cylinders are located by thesurface of the sea, a bell in the form of a receptacle is arranged inthe sea at a predetermined depth below the surface. The bell has anopening in its lower portion, where water may flow in and out. At itsupper portion, the bell is connected in a communicating manner to thepressure side of the compensating cylinder by means of aninterconnecting pipe/hose/conduit. Said interconnecting pipe is alsoconnected to a compressor or other pressure source designed to supplythe bell with air or another fluid having a significantly lower densitythan water.

By introducing such relatively light fluid to the pipe and the pressurebell, a fluid pressure is established, at the surface in theinterconnecting pipe and the pressure sides of the compensatingcylinders, which corresponds to the hydrostatic pressure difference inthe water outside the bell and in the fluid in the bell.

In another embodiment, in which the telescopic section of the riser andthe compensating cylinders are located at a considerable depth below thesurface of the sea, the pressure sides of the cylinders are suppliedwith seawater through an opening in the cylinders, or with another fluidthrough an interconnecting pipe to the surface. The depressurised sidesof the cylinders are connected to the surface via an interconnectingpipe that is under vacuum or filled with a relatively light fluid.

The operation of the arrangement is explained in greater detail in thespecific part of the description with reference to the appendeddrawings.

An advancement of the invention may be to provide a separatingtank/cylinder in the fluid supply for the cylinder, designed to supplythe cylinder with a fluid that is different from the pressure fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The following describes a non-limiting example of a preferred embodimentillustrated in the accompanying drawings, in which:

FIG. 1 schematically shows a floating installation where the hydrostaticpressure of the sea is utilised to pressurise a hydraulic cylinder-likeactuator formed by the through bore of the installation forming acylinder casing and the riser forming the piston rod of the cylinder;

FIG. 2 shows the same as FIG. 1, but here the floating installation isequipped with compensating cylinders of a conventional type; and

FIG. 3 schematically shows an installation where the telescopicconnection of the riser is arranged at a considerable depth below thesurface of the sea, and where several hydraulic cylinders extend betweenand are connected to the upper and lower portions of the riser.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, reference number 1 denotes a riser of a type that isknown per se, which is anchored to the seabed 2. The riser 1 rises up toa normally floating installation 4 located on the surface 6. In apreferred embodiment, see FIG. 1, the floating installation 4 is made upof a buoy equipped with a through bore 8, and where a gable plate 10 issealingly connected to the lower portion of the bore 8. The gable plate10 is equipped with a packing 11 that movably seals against the externaltubular surface of the riser 1. The bore 8 may be provided with asimilar gable plate 12 and packing 13 at its upper end portion. Theriser 1, in a position slightly above the gable plate 10, is equippedwith a piston 14 that movably seals against the bore 8 with the aid of apacking 16.

The bore 8, gables 10, 12, the piston 14 and the riser 1 form ahydraulic cylinder-like actuator 19, in which the cylinder volumebetween the piston 14 and the gable 10 constitutes the pressure side 18of the cylinder 19, while the cylinder volume between the piston 14 andthe gable 12 constitutes the depressurised side 17 of the cylinder.

The pressure side 18 of the cylinder 19 is connected via apipe/hose/conduit 22 to a bell 20 located at a considerable distancebelow the surface of the sea 6. A pipe 24 is connected to a shut-offvalve 26 and joined to the pipe 22. The depressurised side 17 of thecylinder 19 is connected to atmosphere via an opening 27.

When the load of the riser 1 is to be taken up by the buoy 4, a fluidthat is considerably lighter than water is pumped in through the valve26 and the pipes 22 and 24 to the cylinder chamber 18 and the bell 20.When a sufficient amount of fluid has been pumped in, a surface 26 formsin the bell 20, the lower portion of which bell has an opening 28. Thevalve 26 is then closed. The fluid pressure in the pipe 22 at thesurface 6 corresponds to the hydrostatic pressure difference in thewater outside the bell 20 and in the fluid in the bell and the pipe 22.The surface area of the piston 14 is adjusted so as to transfer therequired tensile force to the riser 1.

When the buoy 4 rises, the volume on the pressure side 18 is reduced,whereby fluid flows out of the cylinder 19 and down through the pipe 22to the bell 20. The surface 26 in the bell 20 only falls slightly, dueto the large cross sectional area of the bell 20 relative to that of thepiston 14. When the buoy 4 sinks, pressure fluid flows back to thepressure side 18 of the cylinder 19, thus maintaining a substantiallyconstant force in the riser 1.

The invention is equally suited when using conventional compensatingcylinders 29 between the floating installation 4 and the riser 1, seeFIG. 2.

In another embodiment, the telescope section 38 of the riser 1 islocated at a considerable depth below the surface of the sea 6, see FIG.3. One or more compensating cylinders 40 of a type that is known per se,but which are adapted to subsea use, are connected to the riser 1 aboveand below the telescope section 38 and designed to establish andmaintain a tensile force in the riser 1. The pressure sides 42 of thecompensating cylinders 40, located on one side of the pistons 41 of thecylinders 40, communicate with the seawater on the outside of thecylinders 40 through openings 44, while the depressurised sides 46 ofthe cylinders 40 communicate with the atmosphere by means of a pipe 48and opening 49.

The pipe 48 is filled with a fluid that is significantly lighter thanwater. Alternatively, the pipe is under vacuum. The operation of thecompensating cylinder 40 during the vertical movements of the floatinginstallation is analogous to that described above.

The arrangement according to the invention allows a major simplificationof the heave compensating means of a floating installation 1, theutilisation of the sea's own hydrostatic pressure essentially makingknown pumping and accumulator systems superfluous. Thus a reduction inoperating costs and improved operational reliability may be expectedwhen using the new technique.

What is claimed is:
 1. An arrangement for transferring loads by means ofone or more hydraulic cylinder-like actuators (19, 26, 40) in order toestablish and maintain tension in a riser (1) of the type typically usedin offshore petroleum production, where the riser (1) constitutes aconnection between the sea bed (2) and an installation (4) on or nearthe surface of the sea (6), and where the required pressure differencebetween the pressure side(s) (18, 42) and the depressurised side(s) (17,46) of the actuator(s) (19, 29, 40) is constituted by a hydrostaticpressure difference, characterised in that the pressure side(s) (18) ofthe actuator(s) (19, 29) is/are communicatingly connected with theseawater, directly or indirectly, via a pipe connection (22), an opening(28) being located at a considerable depth below the surface of the sea(6), and where the pipe connection (22) is substantially filled with afluid with a lower density than that of water.
 2. An arrangement inaccordance with claim 1, characterised in that the depressurised side(s)(17) of the actuator(s) (19, 29) is/are communicatingly connected withthe atmosphere through an opening (27).
 3. An arrangement in accordancewith claim 1, characterised in that the depressurised side(s) (17, 46)of the actuator(s) (19, 29, 40) is/are closed and under vacuum.
 4. Anarrangement in accordance with claim 1, characterised in that the pipeconnection (22) communicates with the seawater via a bell (20).
 5. Anarrangement for transferring loads by means of one or more hydrauliccylinder-like actuators (19, 26, 40) in order to establish and maintaintension in a riser (1) of the type typically used in offshore petroleumproduction, where the riser (1) constitutes a connection between the seabed (2) and an installation (4) on or near the surface of the sea (6),and where the pressure sides (42) of the actuators (40) arecommunicatingly connected with the seawater, directly or indirectly, viaan opening (44), where the opening (44) is located at a considerabledepth below the surface of the sea (6), characterised in that thedepressurised sides (46) of the actuators (40) are communicatinglyconnected with the atmosphere via a pipe connection (48) and through anopening (27), and where the pipe connection (22) is substantially filledwith a fluid with a lower density than that of water.
 6. An arrangementin accordance with claim 5, characterised in that the depressurisedside(s) (46) of the actuator(s) (40) is/are closed and under vacuum.